Replication competent Cas-Br-M ecotropic murine leukemia virus (MuLV) induces a wide spectrum of hematopoietic neoplasms after infection of newborn mice. They include T and B cell lymphomas, myelogenous leukemias and erythroleukemias. The mechanisms responsible for this diversity of effects were shown to include the secondary production of lineage specific MuLV as well as effects of interleukin 3 (I1 3). The lineage specific viruses were a) a new transforming virus for murine fibroblasts and pre-B cells, termed Cas-NS-1, and b) a mink lung cell focus-forming (MCF) MuLV, termed Cas-NS-6. Continuous cell lines were established in the presence but not the absence of I1 3 from spleens of mice with lymphomas induced by Cas-Br-M. A new mutation to ashen in B10.F mice was found to be associated with a new germ line integration of an ecotropic provirus. Genetic studies of the mutant mice showed that the coat color mutation and virus reintegration were independent events. Studies of thymic lymphomas induced in AKR mice by either of two MCF MuLV were markedly heterogeneous in their cell-surface antigenic phenotypes. Single MCF MuLV therefore do not predictably induce neoplasms of an antigenically distinct subset of thymocytes.